Semi-permeable membranes with an internal discriminating region

ABSTRACT

The invention is a semi-permeable membrane which comprises a polymeric matrix with two porous surfaces and a region which functions to separate one or more gases from one or more other gases. 
     The membranes of this invention exhibit excellent separation factors and flux. Such membranes are less prone to being damaged due to handling and exposure to contaminants due to the internal region which affects the separation, as the porous surfaces function to protect such region.

BACKGROUND OF THE INVENTION

This invention relates to novel semi-permeable membranes useful forseparating one or more gases from one or more other gases.

In various industries, it is necessary or highly desirable to separateone component from another in a gaseous stream. Processes used toperform such separations include pressure swing absorption, cryogenics,and membrane separations. In a membrane separation, a gaseous streamcontaining the components to be separated is contacted with a membrane,wherein the membrane separates two regions in a manner such that onlythose materials which permeate through the membrane can communicate fromone region to the other. Such membranes are semi-permeable, in that oneor more component of the gaseous mixture selectively permeates throughthe membrane at a rate much higher than one or more of the components inthe gaseous stream. Among such separations are the separation of oxygenfrom nitrogen, and carbon dioxide from methane. The gaseous mixture iscontacted with the membrane in a manner such that the selectivelypermeable species is preferentially transported through the membrane tothe other region. The component which is non-selectively permeable maypermeate through the membrane but at a much slower rate than theselectively permeable species. It is this difference in rates ofpermeation which is used to separate the gaseous species or reduce theconcentration of the less selectively permeated species in the region towhich the permeating gases permeate, or decrease the concentration ofthe more selectively permeated gas in the region from which thepermeating gases permeate.

In such separations, the relative rate of permeation, that is, thedifference in rate of permeation between the selectively permeating gasand the non-selectively permeating gas, is a major factor in theseparation achieved. The higher the ratio of permeation of theselectively permeable gas over the non-selectively permeable gas, thebetter the membrane will perform. Therefore, it is desirable to have ashigh a ratio as possible.

Another important property of membranes is the permeability of the gasesthrough the membrane. If the permeability is too low, the membrane maynot provide enough flow through the membrane to be economical forseparations. Some potential candidates for membrane separations providegood separation factors but low permeabilities for dense membranes. Fluxis the volumetric flow of gas through a particular membrane for a unitarea and time, and indicates the productivity of the membrane. Theseparation factor is the ratio of the permeabilities of the selectivelypermeating species over the non-selectively permeating species. Onetechnique used to improve the flow of the permeating gases through themembrane is to form asymmetric membranes from such polymers. Asymmetricmembranes comprise a membrane with a thin dense region wherein theseparation is effected, and a larger region which is porous throughwhich gases pass with little resistance which provides support for thethin dense layer. The discriminating region is much thinner than a solidor homogeneous membrane can be, as the porous layer provides thestructural integrity of the membrane and supports the thin dense, layer.This thin, dense layer is located on one surface of the membrane. Theformation of an asymmetric membrane with good separation factors andpermeabilities is a difficult chemistry and engineering problem. As thethin, dense layer is on one of the surfaces of the membrane, this thin,dense layer is subject to being damaged by handling or exposure tocontaminants. This damage can result in leaks in the membrane and renderthe membrane less effective in separating gases.

Presently, membranes derived from acetate esters, for example cellulosediacetate, and cellulose triacetate, polyamides, polyimides, andolefins, for example polyethylene, polypropylene,poly-4-methylpentene-1, are used for gas separations. Recently it hasbeen discovered that bisphenol based polycarbonates, andpolyestercarbonates wherein at least 25 percent by weight of thebisphenol moieties are tetrahalogenated, wherein the halogen is Cl orBr, exhibit excellent separation factors for the separation of oxygenfrom nitrogen, but exhibit low flux in the dense form.

What are needed are membranes with regions capable of separating one ormore gases from one or more other gases which have both good separationfactors and flux. What are further needed are membranes which have suchregions which are not subject to damage due to handling or exposure tocontaminants. What are further needed are membranes which exhibit goodphysical properties.

SUMMARY OF THE INVENTION

The invention is a semi-permeable membrane which comprises a polymericmatrix with two porous surfaces and a region which functions to separateone or more gases from one or more other gases.

The membranes of this invention exhibit excellent separation factors andflux. Such membranes are less prone to being damaged due to handling andexposure to contaminants due to the internal region which affects theseparation, as the porous surfaces function to protect such region.

DETAILED DESCRIPTION OF THE INVENTION

The membranes of this invention have two porous surfaces. Moreparticularly, the membranes have two porous regions which start at thesurface of the membrane and continue for some distance into themembrane. Such porous regions are capable of passing the desired gasesto separate through such regions without much resistance. The pores onthe surfaces are large enough such that gases freely pass through themwithout any resistance. Preferably, the pores on the surfaces arebetween about 250 and 10,000Å. In the embodiment wherein the membrane isa hollow fiber, the inner surfaces preferably have pores of from about250 to about 10,000Å, and the outer surfaces preferably have pores offrom 250 to 3,000Å.

The membranes of this invention comprise a porous layer on both sides ofthis membrane, i.e., both the exterior and the interior of a hollowfiber, with an interior region which is discriminating or functions asif it were dense, that is a permeant cannot cross from one surface ofthe membrane to the other without permeating into and through anon-porous or dense region of the membrane. Such a discriminating regionmay be a region of non-continuous porosity. In one embodiment of thehollow fiber form of this membrane the region of non-continuous porosityis located near the lumen of the fiber.

The critical feature of the invention is that such membranes function toseparate one or more gases from one or more other gases. Preferably suchmembranes have an internal region which functions to separate one ormore of the gases contacted with the membranes from one or more of theother gases contacted with the membranes. This region may be a denseregion, a region of non-continuous porosity, or a region which resemblesa closed cell foam.

The membranes of this invention may be prepared from any polymericmaterial which has inherent properties which pass one or more gasesthrough its bulk phase at a faster rate than one or more other gases.Those skilled in the art would recognize which polymeric materials wouldbe suitable. Preferable polymeric materials comprise polyimides,polycarbonates, polyesters, polyestercarbonates, polysulphones,polyethersulphones, polyamides, polyphenylene oxides, and polyolefins.More preferred polymeric materials comprise polyesters, polycarbonates,and polyestercarbonates. Even more preferred polymeric materialscomprise polycarbonates. More preferred polycarbonates are those derivedfrom a bisphenol wherein at least 25 percent of the bisphenol moietiesin the backbone of the polymer are tetrahalogenated wherein the halogenis chlorine or bromine. The polymers useful in this invention should bepolymerized to the extent that the polymers will form a membrane withsufficient mechanical strength to withstand use conditions.

The membranes may be fabricated in any useful membrane form, for exampleflat sheet, hollow fiber, or hollow tube form. The preferred form is thehollow fiber form. These membranes may be used in any form of membranedevice, for example hollow fiber devices, hollow tube devices, spiralwound devices, and plate and frame devices.

The membranes of this invention may be used to separate components in agaseous stream capable of being separated; such separations are wellknown to those skilled in the art. Preferred separations include theseparation of oxygen from nitrogen, nitrogen from methane, carbondioxide from light hydrocarbons especially methane, and helium andhydrogen from other gases such as light hydrocarbons. The preferredseparation is the separation of oxygen from nitrogen.

The membranes of this invention may be prepared by the followingprocess. A mixture of the polymeric material, a solvent for thepolymeric material, and a non-solvent for the polymeric material isprepared. Such mixture is preferably homogeneous at extrusiontemperatures. The mixture should be sufficiently viscous to retain itsintegrity until the membrane is formed. It is preferable that themixture is close to the phase boundary between a one- phase mixture anda two-phase mixture, so the concentrations of the components should bechosen such that the mixture is near the boundary. If the non-solventconcentration is too low, the discriminating region will form on onesurface of the membrane. If the non-solvent concentration is too high,the mixture will not be homogeneous and the membrane may have poreswhich communicate through the membrane. The polymer concentration shouldbe high enough such that the mixture is sufficiently viscous to extrudeand retain its shape at under extrusion conditions. If the polymerconcentration is too high, the discriminating region will be too thickand the pores will be too small, thus reducing the flux through theformed membrane. The mixture is heated to, or above, the extrusiontemperature. The temperature for the extrusion is that temperature suchthat the mixture has sufficient viscosity for extrusion, and whichfacilitates phase inversion of the material when exposed to theconditions of the quench zone or zones. The polymer mixture is extrudedthrough a die of the desired shape into and through one or more quenchzones, wherein one of the quench zones comprises a liquid which is asolvent for the solvent and non-solvent, and which has very lowsolubility in the polymeric material. The process is performed underconditions such that the polymer mixture undergoes phase inversion inone or more of the quench zones, wherein a phase rich in the polymericmaterial and a phase rich in the solvent and the non-solvent are formed,and the solvent and non-solvent is removed from both phases.

The preferred polycarbonates useful in this invention are derived frombisphenols wherein a significant portion of the bisphenols used toprepare the polycarbonates are tetrahalosubstituted; more preferably thetetrahalo substituents are found in the 3,5-positions on the aromatic orphenolic rings. The presence of a significant portion of the residue oftetrahalo bisphenols enhance the properties of membranes that areprepared therefrom. More particularly, such membranes have enhancedseparation factors with respect to oxygen/nitrogen, hydrogen/methane,and carbon dioxide/methane separations.

More preferably the polycarbonates useful in this invention comprisepolymers with backbone units which correspond to the formula ##STR1##wherein R at each occurrence is independently H, Cl, Br, or C₁ -C₄alkyl; and, R¹ is carbonyl, --S--, --SO₂ --, --O--, a c₁ -C₆ divalenthydrocarbon, a C₁ -C₆ divalent halocarbon radical, or an inertlysubstituted C₁ -C₆ hydrocarbon radical, with the proviso that at least25 weight percent of the bisphenol moieties in Formula I bear R groupswhich are exclusively Br, Cl, or mixtures thereof.

Preferably, at least 35 weight percent of the bisphenol moieties in thepolycarbonate backbone bear R groups which are exclusively bromine,chlorine, or mixtures thereof. More preferably, at least 50 weightpercent of the bisphenol moieties in the backbone bear R groups whichare exclusively bromine, chlorine, or mixtures thereof. Even morepreferably, at least 75 weight percent of the bisphenol moieties in thepolycarbonate backbone bear R groups which are exclusively bromine,chlorine, or mixtures thereof. Even more preferably, the polycarbonateis derived from bisphenols where R is exclusively bromine, chlorine, ormixtures thereof. In the embodiment wherein the polycarbonate isprepared from tetrachlorobisphenols, it is preferable that thepolycarbonate backbone contain about 90 percent by weight or greaterunits derived from tetrachlorobisphenols, more preferably 95 percent byweight, and more preferably 100 percent by weight. Bromine is thepreferred halogen herein. Examples of preferred bisphenols which bear Rgroups which are exclusively Br or Cl are2,2-bis(3,5-bromo-4-hydroxy-phenyl)propane and2,2-bis(3,5-chloro-4-hydroxy-phenyl)propane with2,2-bis(3,5-bromo-4-hydroxy-phenyl)propane being most preferred.Preferably those R groups which are not halogen are methyl or hydrogen,and most preferably hydrogen.

In the hereinbefore presented formulas, R is preferably chlorine,bromine, hydrogen or C₁₋₄ alkyl, more preferably chlorine, bromine,hydrogen, or methyl, even more preferably chlorine and bromine, and mostpreferably bromine. R¹ is preferably a C₁₋₆ divalent hydrocarbon, morepreferably a C₁₋₆ alkylidene moiety, even more preferably anisopropylidene moiety.

The polycarbonates useful in this invention can be prepared by anyprocess known in the art which prepares polycarbonates with suitableproperties for membrane formation. See Encyclopedia of Polymer Science &Technology, Editor Mark et al, Interscience Division of John Wiley &Sons, N.Y., N.Y., 1969, Vol. 10, pages 714-725 (relevant portionsincorporated herein by reference). The polymers useful in this inventionshould be polymerized to the extent that the polymers will form amembrane with sufficient mechanical strength to withstand useconditions.

In one preferred embodiment the halogenated bisphenol basedpolycarbonate membranes are prepared by the process which comprises:

(A) forming a mixture comprising

(i) a bisphenol polycarbonate wherein at least 25 percent by weight ofthe bisphenol moieties are tetra halogenated wherein the halogen ischlorine or bromine;

(ii) a solvent for the polycarbonate which comprises a glycol etherwhich corresponds to the formula R³ O--(CH₂ CH₂ O)_(r) --R³ wherein R³is methyl or ethyl, and r is an integer of between about 1 and 20 ; adialkyl ketone wherein the alkyl groups independently are methyl orethyl; morpholine substituted on the nitrogen atom with an alkyl, formylor alkanoyl moiety; pyrrolidinone or N--C₁₋₄ alkyl, N--C₅₋₆ cycloalkyl,or N--C₆₋₁₀ aryl or alkaryl substituted pyrrolidinone; C₁₋₄alkoxycarbonyl, formyl, nitro, or halo substituted benzene;tetrahydrofuran; dimethyl formamide, cyclohexanone; N,N-dimethylacetamide; acetophenone; caprolactone; methylene chloride; sulfolane;cyclohexyl acetate; 1,1,3,3,-tetramethylurea; isophorone;1-formyl-piperidine; methyl salicylate; hexamethylphosphoramide; phenylether; or bromonaphthalene; and,

(iii) a non-solvent for the polycarbonate which comprises a glycol orglycol ether which corresponds to the formula R⁴ O--(CH₂ CH₂ O)_(q) --R⁴wherein R⁴ independently in each occurrence is hydrogen or C₁₋₄ alkyl,and q is an integer of about 1 to about 250; an ester corresponding tothe formula R⁵ COOR⁶ wherein R⁵ is hydrogen or C₁₋₁₉ alkyl, and R⁶ isC₁₋₁₀ alkyl; a C₁₋₁₀ alkanol; cyclo-hexane, unsubstituted or substitutedwith an alkyl, cycloalkyl, or perfluoroalkyl moiety; a C₅₋₂₀ alkane; adialkyl ketone wherein at least one of the alkyl moieties is C₃ orgreater; an amide corresponding to the formula R⁷ CONHR⁸ where in R⁷ ishydrogen or C₁₋₁₀ alkyl and R⁸ is C₁₋₁₀ alkyl; an acetyl or C₁₋₁₀ alkylnitrile; acetone; a C₁₋₁₀ alkyl aldehyde; a trialkyl amine;nitromethane; trialkyl orthoformate; diacetone alcohol; dimethylmalonate; decahydronaphthalene; tetrahydronaphthalene; malononitrile;dicyclophexyl; ethylene carbonate; sulfolane; alkyl or cycloalkylsubstituted benzene; or water;

(B) heating the mixture to a temperature at which the mixture forms ahomogeneous fluid and is extrudable;

(C) extruding the heated mixture into a shape suitable for membrane use;and

(D) passing the formed membrane through one or more quench zones,wherein the mixture phase separates, and the major portion of thesolvent and non-solvent are removed from the formed membrane wherein oneof such quench zones comprises a liquid which has a very low solubilityin the polycarbonate,;

wherein the membrane formed has a porous outer and inner surface with adiscriminating region capable of separating oxygen from nitrogen.

The polycarbonate mixture may be extruded into any shape which is usefulas a membrane. Such shapes include flat sheets, hollow tubes, and hollowfibers. The most preferred shape is the hollow fiber shape. The processfor preparing this preferred shape may be described as follows. Thefollowing description of the process with respect to the formation ofhollow fiber membranes refers to one fiber, but the process may beperformed on one fiber at a time or a multitude of fiberssimultaneously. In fact, most hollow fiber preparation processes involveforming several fibers and processing them simultaneously. Thedescription shall be understood to include forming and processing onefiber or a multitude of fibers simultaneously.

A process for preparing a hollow fiber comprising a tetrahalogenatedbisphenol polycarbonate which comprises:

(A) forming a mixture comprising

(i) a bisphenol polycarbonate wherein at least 25 percent by weight ofthe bisphenol moieties are tetrahalogenated wherein the halogen ischlorine or bromine;

(ii) a solvent for the polycarbonate as described hereinbefore; and,

(iii) a non-solvent for the polycarbonate as described hereinbefore;

wherein the mixture has a sufficient viscosity to allow extrusion attemperatures at which the mixture is homogeneous;

(B) heating the mixture to a temperature at which the mixture forms ahomogeneous fluid and is extrudable;

(C) extruding the heated mixture into a hollow fiber form;

(D) passing the formed fiber through one or more quench zones whereinthe mixture phase separates, and the major portion of the solvent andnon-solvent are removed from the formed fiber, while a core fluid ispassed down the hollow core of the fiber under conditions sufficient toprevent the fiber from collapsing, wherein one of the quench zonescomprises a liquid which has low solubility in the polycarbonate; and,

wherein the fiber formed has a porous inner and outer surface and thefiber is capable of separating oxygen from nitrogen.

Preferably, the polymer solvent non-solvent mixture has a viscosity atextrusion temperatures of about 10,000 to about 200,000 poise in theembodiment wherein the core fluid is a gas, and more preferably betweenabout 30,000 and 45,000 poise. The viscosities described herein arebased upon rheometric measurements taken at 82° C. at a frequency of 1radian per second. Preferably, the polymer used to prepare the membraneshas a molecular weight (M_(w)) of 100,000 or greater, more preferablybetween 100,000 and 300,000.

Discriminating region refers to a region which functions to separate oneor more gases from one or more other gases, and may be a non-porousregion or the equivalent of a non-porous region, for example, a regionon non-continuous porosity. "Homogeneous fluid" as used herein refers toa fluid which is a mixture of components and which is in one phase.Extrusion refers herein to passing a fluid of the polymer mixturethrough a die to form the fluid into the desired shape. "Extrudable" asused herein refers to a material which is capable of extrusion to form adesired shape, wherein the material formed to such shape once formedretains such shape. "Quench" as used herein refers to exposing thepolymer mixture to conditions such that the polymer mixture partially orcompletely undergoes a phase separation. "Phase separation" refersherein to the phenomena wherein the polymer mixture undergoes separationinto a polymer rich phase and a solvent-non-solvent rich phase."Leaching" as used herein refers to the phenomena wherein entrainedsolvent and non-solvent liquids are removed from the polymer rich phase.

The polymer mixture, which is extruded to form the membranes of thisinvention, comprises the polycarbonate described hereinbefore, a solventfor the polycarbonate, and a non-solvent for the polycarbonate. Thesolvent functions to dissolve the polymer and the non-solvent into ahomogeneous solution at the temperatures used for extrusion so that themixture may be extruded. The non-solvent functions to aid in theformation of pores in the polymer when it undergoes phase separation inthe quench zone.

An optional fourth component, a dissolving medium, may be added to thepolymer mixture to aid in the formation of a homogeneous mixture. Thedissolving medium is used to enhance the dissolution of the polymer intothe solvent non-solvent mixture. Usually the dissolving medium isremoved from the mixture prior to the extrusion, usually by flashing itoff.

The solvent may be any solvent for the polymer from which the membranesare to be formed, which dissolves enough of the polymer so as to form asolution viscous enough to be extrudable at the extrusion temperatures.The amount of solvent used depends upon the polymer used, thenon-solvent used, the desired properties of the membrane, and the methodof quenching the fiber.

In the embodiment wherein the polymer is a tetrahalosubstitutedbisphenol based polycarbonate, the following solvents are preferred. Theglycol ethers useful as a solvent for the polycarbonate corresponds tothe formula R³ O--(CH₂ CH₂ O)_(r) R³ wherein R³ is methyl or ethyl, andr is an integer of between about 1 and 20. Preferably, r is an integerof between about 1 and about 10 , and even more preferably between about1 and about 4, most preferably when R³ is methyl r is between about 1and about 4, and when R³ is ethyl r is between about 2 and about 4.Examples of such glycol ethers include ethylene glycol dimethyl ether,diethylene glycol dimethyl ether, and bis(2-methoxy-ethyl ether).Preferred dialkyl ketones useful as solvents for the polycarbonatesinclude dimethyl ketone, diethyl ketone, and methyl ethyl ketone.Preferred substituted morpholines are those with a C₁₋₁₀ alkyl, formylor C₁₋₁₀ alkanoyl moiety substituted on the nitrogen atom; morepreferred are those with a C₁₋₄ alkyl, formyl or C₁₋₄ alkanoyl moietysubstituted on the nitrogen atom. Examples of substituted morpholinesinclude N-formylmorpholine and N-ethylmorpholine. Preferredpyrrolidinones useful as solvents include pyrrolidinone, N-methylpyrrolidinone, N-ethyl pyrrolidinone, N-cyclophexyl pyrrolidinone,N-benzyl pyrrolidinone, and N-phenyl pyrrolidinone; with N-methylpyrrolidinone and N-ethyl pyrrolidinone more preferred; and N-methylpyrrolidinone most preferred. The term pyrrolidinone as used hereinrefers to compounds named as pyrrolidinones and pyrrolidones. Preferredsubstituted benzenes useful as solvents for the polycarbonatescorrespond to the formula: ##STR2## wherein R⁹ is C₁₋₄ alkoxycarbonyl,nitro, halo or a formyl moiety; and b is an integer of about 1 to about6, with the proviso that wherein R⁹ is alkoxycarbonyl b is 1 . Thepreferred halogens are chlorine and bromine, with chlorine mostpreferred. Preferably, b is between about 1 and 3. Examples ofsubstituted benzenes useful as solvents include chlorobenzene,dichlorobenzene, benzaldehyde, nitrobenzene, ethyl benzoate, methylbenzoate, and 1,2,4-trichlorobenzene.

Preferred solvents comprise N-methylpyrrolidinone, tetrahydrofuran,ethylene glycol dimethylether, diethylketone, N-ethylmorpholine,dimethylformamide, cyclohexanone, bis(2-methoxyethylether),N,N-dimethylacetamide, acetophenone, methylene chloride, or sulfolane.More preferred solvents include N-methylpyrrolidinone, ethylene glycoldimethylether, tetrahydrofuran, diethylene glycol dimethylether,acetophenone, methylene chloride, or cyclohexanone. The most preferredsolvent is N-methylpyrrolidinone.

The non-solvent may be any compound which does not substantiallydissolve the polymer from which the membrane is to be prepared atextrusion temperatures, is soluble in the solvent, and which aids in theformation of pores in the polymer rich phase when the spin mixture isextruded into a quench zone. The amount of non-solvent used depends uponthe polymer used, the solvent used, the desired properties of themembrane, and the method of quenching the fiber. The particularnon-solvents useful for each polymer are well known to those skilled inthe art or readily ascertainable by those skilled in the art.

The following non-solvents are preferred for the polycarbonates. Theglycols and glycol ethers useful as non-solvents for the polycarbonatecorrespond to the formula R⁴ O-(CH₂ CH₂ O)_(q) -R⁴ wherein R⁴ isindependently in each occurrence hydrogen or C₁₋₄ alkyl, and q is aninteger or about 1 to about 250. Preferably R⁴ is hydrogen. Preferably qis an integer of about 2 to about 100, more preferably of about 3 toabout 60, and most preferably about 3 to about 15. Examples of preferredglycols and glycols ethers include 2-ethoxyethanol, polyethylene glycolswith molecular weights of up to about 1450, triethylene glycol,diethylene glycol, diethylene glycol dibutylether. Esters useful asnon-solvents correspond to the formula R⁵ COOR⁶ wherein R⁵ is hydrogenor C₁₋₁₉ alkyl, and R⁶ is C₁₋₁₀ alkyl. Preferably R⁵ is hydrogen or C₁₋₄alkyl, and R⁶ is C₁₋₄ alkyl. Most preferably, R⁶ is ethyl or methyl.Examples of preferred esters include methyl formate, ethyl formate,methyl acetate, n-octyl acetate, methyl laurate, methyl myristate, butylstearate, and methyl stearate. Preferred alkanols useful as non-solventsinclude methanol, ethanol, 2-propanol, and 1-hexanol. Preferredcyclophexanes useful as non-solvents include those which areunsubstituted or substituted with a C₁₋₁₀ alkyl, C₃₋₁₀ cycloalkyl orC₁₋₄ perfluoroalkyl moiety, More preferred cyclohexanes useful asnon-solvents include those which are unsubstituted or substituted with aC₁₋₄ alkyl, C₅₋₆ cycloalkyl or trifluoromethyl moiety. Examples of suchcyclohexanes include cyclohexane, methylcyclohexane,isopropylcyclophexane, t-butyl-cyclohexane and dicyclohexyl. PreferredC₅₋₂₀ alkanes useful as non-solvents include hexane, dodecane, andhexadecane. Preferred dialkyl ketones useful as non-solvents for thepolycarbonates include those wherein one of the alkyl moieties is C₃₋₁₀and the other C₁₋₁₀. Examples of preferred dialkyl ketones useful fornon-solvents include methyl isobutyl ketone, and diisopropyl ketone.Preferred amides useful as non-solvents include those amidescorresponding to the formula R⁷ CONHR⁸ wherein R⁷ is preferably hydrogenor C₁₋₃ alkyl, and R⁸ is preferably C₁₋₄ alkyl. Examples of preferredamides include N-methyl formamide, and N-methyl acetamide. Preferablenitriles include acetyl and C₁₋₃ alkyl nitriles. Examples of preferrednitriles include acetonitrile, and propionitrile. Preferred aldehydesare C₁₋₄ alkyl aldehydes, with butyraldehyde most preferred. Preferredsubstituted benzenes include formyl, alkyl, and cycloalkyl substitutedbenzenes which correspond to the formula ##STR3## wherein R¹⁰ is C₁₋₁₀alkyl, C₃₋₁₀ cycloalkyl, or formyl, and b is as defined hereinbefore.Preferably, R¹⁰ is C₁₋₄ alkyl, C₅₋₆ cycloalkyl, or formyl.

Preferred non-solvents for the polycarbonates include triethyleneglycol, 2-ethoxyethanol, diethylene glycol dibutyl ether, polyethyleneglycols with molecular weights of up to about 1450, diethylene glycol,dodecane, hexadecane, cyclohexane, methylcyclohexane, perchloroethylene,diisopropylketone, isopropylketone, isopropylcyclohexane,t-butylcyclohexane, N-methylformamide, decylene, N-methylacetamide,tetralin, dicyclohexyl, cyclophexyl benzene, diethylene glycoldibutylether, carbon tetrachloride, or water. More preferrednon-solvents for the polycarbonates include water, diisopropylketone,tetraethylene glycol dimethylether, diethylene glycol dibutyl ether,hexadecane, diethylene glycol, triethylene glycol, polyethylene glycolwith molecular weights of up to about 1450, 2-ethoxyethanol, carbontetrachloride, or dodecane. The most preferred non-solvents for thepolycarbonates are triethylene glycol, and polyethylene glycols withmolecular weights of up to about 400.

Some compounds may be both a solvent and a non-solvent, wherein itsfunction is dictated by the temperature at which the membrane is formed.

In some embodiments, a solubilizing agent is used to aid in preparing ahomogeneous polymer mixture. The solubilizing agent may be any solventwhich aids in preparing a homogeneous polymer mixture. The solubilizingagent is preferably a solvent which has a boiling point lower than theextrusion temperature and the boiling points of the solvent andnon-solvent. The polymer mixture may be formed at temperatures below theextrusion temperature and the solubilizing agent aids in forming ahomogeneous mixture at such temperatures. Preferably the solubilizingagent flashes off, or is removed, prior to extrusion. Preferredsolubilizing agents for the polycarbonate based mixtures includehalogenated hydrocarbons, cyclic and non-cyclic ethers, and alkylketones. More preferred solubilizing agents include methylene chloride,tetrahydrofuran, methyl ethyl ketone, methyl iodide, and chloroform. Amost preferred solubilizing agent is methylene chloride.

The solubilizing agent is not used in embodiments where the mixture ismixed at elevated temperatures under high shear or with good mixing,where the mixture is to be extruded shortly after formation of thehomogeneous mixture.

Certain solvents and non-solvents may cause degradation of the polymerif the polymer mixture is maintained at elevated temperatures forextended periods of time. The solvent and non-solvent should be chosento be compatible; in particular the non-solvent must be soluble in thesolvent, and the non-solvent must be capable of forming pores in thequenched polymer in the presence of the solvent. Skilled artisans oftendescribe the solvent and non-solvent as a solvent non-solvent pair.Preferred solvent non-solvent pairs for the polycarbonates includeN-methylpyrrolidinone and triethylene glycol, N-methylpyrrolidinone anda polyethylene glycol with a molecular weight of up to about 1450,ethylene glycol dimethylether and water, tetrahydrofuran and water,ethylene glycol dimethylether and diisopropylketone, tetrahydrofuran anddiisopropylketone, diethylene glycol dimethylether and water, diethyleneglycol dimethylether and tetralin, tetraetylene glycol dimethylether andN-methyl-acetamide, acetophenone and diethylene glycol dibutylether,methylene chloride and carbon tetrachloride, cyclohexanone and dodecane,and acetophenone and hexadecane. More preferred solvent non-solventpairs for the polycarbonates are N-methylpyrrolidinone and triethyleneglycol, cyclohexanone and dodecane, N-methylpyrrolidinone and apolyethylene glycol with a molecular weight of up to about 400, andacetophenone and hexadecane. The most preferred solvent non-solventpairs for the polycarbonates are N-methylpyrrolidinone and triethyleneglycol, and N-methylpyrrolidinone and a polyethylene glycol with amolecular weight of up to about 400.

The polymer mixture should comprise appropriate amounts of the polymer,solvent, and non-solvent to be extrudable at the extrusion temperaturesand to form the membranes of this invention. In particular, the solutionshould have an acceptable viscosity for such extrusion at extrusiontemperatures. The upper limit on the viscosity is that viscosity atwhich the solution is too viscous to extrude. The lower limit on theviscosity is that viscosity at which the fiber loses its integrity afterleaving the vicinity of the extrusion die.

Preferably, the spin composition comprises between 30 and 60 percent byweight of the polymer, and 40 and 70 percent by weight of the combinedsolvent and non-solvent. More preferably, the spin composition comprisesbetween 40 and 60 percent by weight of the polymer, and 40 and 60percent by weight of the combined solvent and non-solvent. In theembodiment wherein the polymer is a tetrahalogenated bisphenol basedpolycarbonate the spin composition comprises, even more preferably,between about 44 and 56 percent by weight of polymer and between about44 and 56 percent by weight of a mixture of solvent and non-solvent, andmost preferably, the spin composition comprises between 50 and 55percent by weight of polymer and between about 45 and 50 percent byweight of the combined solvent and non-solvent. The ratio of the solventto the non-solvent is dependent upon the polymer, the solvent and thenon-solvent used and the relative solubilities with respect to oneanother. The solvent and non-solvent are present in a ratio of betweenabout 0.9 and 5.1, more preferably between about 1.8 and 2.7, and mostpreferably between about 2.0 and 2.4.

Prior to extrusion, the mixture is heated to a temperature at which themixture is homogeneous and has an appropriate viscosity for extrusion.The upper limit on the pre-extrusion temperature is that temperature atwhich the polymer undergoes detrimental degradation in the presence ofthe particular solvent and non-solvent. Detrimental degradation meansherein that the polymer degrades sufficiently that the viscosity of thepolymer mixture is significantly lowered below that viscosity at whichacceptable membranes can be formed, or the polymer mixture cannot form amembrane of this invention which is capable of separating oxygen fromnitrogen. In the embodiment wherein the membrane is a hollow fiber andthe core fluid is a gas, this is indicated where the hollow fibercollapses in the quench zone. In the embodiment wherein the polymer is atetrahalosubstituted polycarbonate the preferred upper pre-extrusiontemperatures are about 200° C. or below, more preferred upperpre-extrusion temperatures are about 130° C. or below. This upper limitis significantly affected by the kind of extrusion apparatus that isused. Generally there is a tradeoff between the temperature to which youcan raise the composition and the residence time in the heating area.With lower residence times, the polymer mixture can be heated to highertemperatures. The lower limit on the pre-extrusion temperature is thattemperature at which the viscosity of the spin solution is sufficientlylow enough to allow extrusion. In the embodiment wherein the polymer isa tetrahalosubstituted polycarbonate the preferred lower temperaturesare 50° C. or above, more preferred lower temperatures are 90° C. orabove. Generally, the spin solution is extruded at the temperaturesdescribed hereinbefore with respect to the pre-extrusion heating.Provided the temperature of the polymer mixture during extrusion iswithin the functional limits described hereinbefore, the actualtemperature of extrusion can be significantly lower than thepre-extrusion temperature, for example, as much as 20° C. lower.

The polymer is extruded into one or more quench zones. Such zonesfunction to facilitate phase separation of the polymer mixture, removalof a large portion of the solvent and the non-solvent, and to provide azone where the fiber may be drawn to its final diameter. The quench zonemay comprise one or more zones. At least one of such zones must be aliquid zone which comprises a liquid which has a low solubility in thepolymer from which the membrane is to be formed. Such zones may furthercomprise air quench zones, liquid quench zones, or a combinationthereof. In one embodiment, the extruded polymer mixture may be extrudedinto an air quench zone, and the extruded polymer mixture is thereafterpassed into one or more liquid quench zones. In another embodiment, thepolymer mixture may be extruded directly into a liquid quench zone, andthereafter may be passed into one or more additional liquid quenchzones.

In a preferred embodiment the polymer mixture is extruded into an airquench zone, and thereafter passed into one or more liquid quench zones.In this embodiment the polymer mixture begins to lose a portion of thesolvent and non-solvent due to volatilization and a significant amountof the draw down of the fiber occurs in the air quench zone. Further,the phase separation of the polymer mixture may begin in this zone. Thetemperature and residence time in the air quench zone should besufficient such that there is partial removal of the solvent in thezone, the polymer mixture does not undergo complete phase separation insaid zone, and the fiber undergoes significant draw down during theprocess. If the polymer mixture completely phase separates prior toexiting the air quench zone, a significant amount of solvent andnon-solvent may be entrained in the polymer mixture such that it cannoteasily be removed from the polymer. The upper limit on the temperatureon the air zone is that temperature below which the polymer mixture hassufficient viscosity to retain its shape and integrity. Preferred uppertemperatures are about 90° C. or below, with more preferred uppertemperatures being about 40° C. or below, and the most preferred uppertemperatures being about 25° C. or below. The lower temperature of theair quench zone is that temperature above which the polymer mixtureundergoes substantially complete phase separation while in the airquench zone. Preferred lower temperatures are about 0° C. or above, withmore preferred lower temperatures being about 10° C. or above, and themost preferred lower temperatures being about 20° C. or above. As notedhereinbefore, the temperatures and the residence time are interdependentvariables; at cooler temperatures the residence time is shorter while athigher temperatures the residence time is longer so as to achieve thedesired results in the air quench zone.

The preferred upper limit of the residence time in the air quench zoneis about 10 seconds or less, more preferably 6 seconds or less, and mostpreferably 1 second or less. The lower residence time is preferably 0.1seconds or greater and most preferably 0.25 seconds or greater. If theenvironment in the air quench zone is too humid, damage to the extrudedshape may occur. Preferably, the humidity at about 24° C. is 60 percentor less. A more preferred humidity range is between about 30 and 60percent at 24° C. It may be desirable to place a shroud around the airquench zone so as to cut down variability in the shapes due to undesiredair currents. In some embodiments, it may be desirable to cool theshroud so as to allow better control of the quenching in the air drawzone. In one embodiment it may be preferred to pass a gentle stream ofair in a counter current fashion along the membrane as it is beingextruded.

The speed of extrusion is not critical to the process, provided anacceptable residence time in the quench zones is achieved. Therefore,the line speed may be as fast as the equipment, including the size ofthe quench zones, allows and which results in acceptable properties inthe membranes. Preferably, lower line speeds are 50 feet per minute orabove, with about 150 feet per minute or above preferred. Preferably,upper line speeds are 1000 feet per minute or below, with 500 feet perminute or below preferred.

In that embodiment where hollow fiber membranes are being formed, a corefluid is preferably passed down the core of the hollow fiber to preventthe fiber from collapsing. The core fluid may be any fluid whichprevents the fiber from collapsing and which does not deleteriouslyaffect the membrane properties. The core fluid may be a gas or a liquid,preferably a gas. Preferred core gases may include air, nitrogen, argon,or a gas which enhances the properties of the membrane. The core fluidpressure may be any pressure which prevents the fiber from collapsingand which does not deleteriously affect the membrane properties of thefiber, and is preferably between about 0.1 and 0.5 inches of water, morepreferably 0.25 to 0.4 inches of water.

From the air quench zone, the membrane is passed into one or more liquidquench baths. In the liquid quench baths, the membrane completes phaseseparation and a major portion of the remaining solvent and non-solventare removed. The liquid quench baths can comprise any liquid whichdissolves both the solvent and non-solvent and which does notdeleteriously affect the membrane properties. Furthermore, the liquidused in the quench zones should have a very low solubility in thepolymer; preferably the solubility is about 5.0 percent by weight orlower. More preferably, the quench liquid has a solubility in thepolymer of about 3.0 percent by weight or lower, even more preferably ofabout 1.0 percent by weight or lower, and most preferably of about 0.5percent by weight or lower. Examples of preferred quench liquids includelower alcohols, water, fluorocarbons, lower aliphatic hydrocarbons, ormixtures thereof. The preferred quench bath liquid for thetetrahalosubstututed bisphenol based polycarbonates is water.

Optionally, after leaving the first liquid quench bath, the membrane maybe passed through or contacted with other liquid baths. The conditionsof each bath is dependent upon the number of baths used and theconditions of the other baths. The conditions of the first liquid quenchbath is dependent upon whether other liquid quench baths are used. Ifonly one bath is used, the conditions should be such that the fibercompletes its phase separation, and the majority of the solvent andnon-solvent are removed from the fiber in the bath. Under suchcircumstances, a preferred upper temperature is 90° C. or below and 30°C. or below being most preferred. The preferred lower temperature is 0°C. or above, with 20° C. or above being more preferred. The residencetime under this condition should be sufficient to allow completion ofthe phase separation of the fiber and to allow removal of a significantportion of the remaining solvent and non-solvent. The residence time inthe single bath can be as long as the economics of the process allow.Such residence times may be as long as no deleterious effects resultfrom such residence times, for example damage from bacterial growth.Residence times of up to several days may be used. Preferably, the upperlimit on the residence time is about 30 minutes or lower, morepreferably 10 minutes or lower. Preferably, the lower residence time is2 minutes or greater, more preferably 5 minutes or greater.

In a preferred embodiment, two liquid quench baths are used. In thisembodiment, the quench bath temperature and residence time in the firstquench bath should be sufficient to result in significant phaseseparation of the polymer mixture in said zone, and to allow some of thesolvent and non-solvent to be removed from the fiber. The lower bathtemperature may be the temperature above the freezing point of the bath.Generally, the lower the bath temperature the better the bath functions.Where the bath comprises water the preferred lower temperature is 0° C.or greater. The preferred upper temperature is 30° C. or less, morepreferably 20° C. or less, and most preferably 10° C. or less. The lowerlimit on the residence time is preferably 0.1 seconds or greater, andmore preferably 1.0 second or greater. The upper residence time ispreferably 600 seconds or less, more preferably 300 seconds or less,even more preferably 20 seconds or less, the most preferably 2 secondsor less. The second liquid quench bath functions to remove most of theremaining solvent and non-solvent. The conditions of the second liquidquench bath should be such that most of the solvent and non-solvent areremoved during its presence in the bath. The temperature of the secondliquid quench bath is that temperature which facilitates the removal ofthe solvent and non-solvent from the membrane. The upper temperature isthat temperature at which either the bath remains in the liquid form, orthe fiber properties are deleteriously affected. The lower limit ontemperature is that temperature below which the solvent and non-solventare no longer removed from the polymer mixture at an acceptable rate. Inthe most preferred embodiment wherein the bath comprises water, thepreferred lower temperatures are 70° C. or above with a more preferredlower temperature of 80° C. or above. In this embodiment, preferredupper temperatures are 100° C. or below and more preferred uppertemperatures are 90° C. Generally, as the temperature is lowered, theresidence time required to achieve the same removal of solvent andnon-solvent becomes longer. After the fiber is removed from the one ormore quench baths, the fiber preferably contains 1.2 percent or less ofthe solvent and non-solvent, and more preferably less than 0.5 percentby weight of the solvent and non-solvent.

In the embodiments wherein one or more liquid quench baths are used,after removal from the first liquid quench bath, the fibers are passedover a set of godets and either passed into another bath or taken up.After completion of the processing, the fibers may be stored in a liquidwhich does not deleteriously affect the properties of the fibers. Themost preferred liquid is water.

In the embodiment wherein the membrane is a hollow fiber, the fiber sizeis that fiber size which allows good separation of oxygen from nitrogenwith reasonable flux, and acceptable pressure drops across the fiber.Preferably, the fiber size is between about 175 ×117 (outside diameterOD × inside diameter ID) microns to about 100 ×65 microns and has anOD/ID ratio of about 1.5. In the embodiment wherein the membrane isprepared from a tetrahalosubstituted bisphenol based polycarbonate, themembrane preferably has a separation factor for oxygen and nitrogen of6.0 and greater, more preferably 6.5 or greater, and most preferably 6.8or greater. Preferably, such membrane has a flux of 3.0 ×10⁻⁶ scc/cm²cmHg sec. or greater, more preferably 6.0 ×10⁻⁶ scc/cm² cmHg sec. orgreater, and most preferably 8.0 ×10⁻⁶ scc/cm² cmHg sec. or greater.

Before fabrication of a module, the membrane is preferably dried. Themembrane may be dried by exposing it to the flow of air or an inert gas.Such exposure preferably takes place at a temperature of between about20° C. and about 80° C. Such gas flow may be a gentle flow eithersubstantially perpendicular to the longitudinal direction of themembrane or along the longitudinal direction of the membrane. In anotherembodiment wherein the membrane is in the hollow fiber form, the gas maybe blown down the core during winding of the module. The gas used to drythe membrane may be any gas which is sufficiently dry to aid in theremoval of liquid from the pores and lumen of the membrane. Such gasesinclude nitrogen, argon, and air.

As used herein, the term semi-permeable membrane refers to a membranewhich displays different permeabilities for different species ofmolecules, and therefore may be used in the separation of ions andmolecules having different permeabilities across the membrane. Permeateas used herein refers to those species which permeate through themembrane at a much faster rate than other species. Non-permeate refersherein to those species which permeate at a much slower rate than theother species present.

Preferably, the membrane exhibits permeability properties similar to adense membrane with an effective thickness of about 10μ or less, morepreferably of about 1.5μ or less and most preferably of about 0.5μorless. Effective thickness means herein that the membrane functions as ifit is a homogeneous flat membrane of such thickness.

The membrane of this invention may be used for separating oxygen fromnitrogen by contacting a gaseous stream containing oxygen and nitrogenwith the membrane of this invention under conditions such that oxygenselectively permeates through the membrane, in comparison to nitrogen.Preferably, the membrane is sealingly engaged to a vessel which definesa space communicating with only one side of the membrane, such that thepermeable oxygen contacting the other side of the membrane can permeatethrough the membrane to the non-communicating space, at a significantlyfaster rate than the nitrogen communicates or permeates through themembrane. Preferably, the oxygen and nitrogen are a part of an airstream. Preferably, the pressure on the communicating side of themembrane is between about 40 psia (about 276 kPa) and about 1000 psia(about 6900 kPa), more preferably between about 80 (about 551 kPa) andabout 160 psia (about 1002 kPa). The temperature at which the mixedoxygen and nitrogen stream is contacted with the membrane is preferablybetween about -10 and 80° C., most preferably between about 0 and 45° C.The pressure differential across the membrane is preferably betweenabout 40 psia (about 276 kPa) and about 1000 psia (about 6900 kPa), andmore preferably between about 95 (about 65 kPa) and about 160 psia(about 1002 kPa).

In one preferred embodiment, the membrane is in a hollow fiber form. Inthe embodiment wherein the membrane is in hollow fiber form, it ispreferable to contact the mixed nitrogen and oxygen stream with amembrane on the inside of the hollow fiber under conditions such thatthe oxygen selectively permeates out of the hollow fibers and a streamwhich is rich in oxygen is taken off of the shell side of the membrane.This oxygen enriched stream can be further oxygen enriched by contactingwith one or more membranes in succession. Alternatively, thenon-permeating oxygen depleted nitrogen stream may be further depletedof oxygen by contacting the stream with one or more further membranes insuccession.

SPECIFIC EMBODIMENTS

The following examples are included for illustrative purposes only anddo not limit the scope of the claims or the invention. Unless otherwisestated, all parts and percentages are by weight. In each of the examplesat least four samples are tested for permeation properties. The flux andseparation factor data are reported as an average of all of the sampleswith a standard deviation. Molecular weights as used herein are weightaverage molecular weights measured using narrow molecular weight rangepolystyrene standards.

Examples 1-14 PREFERRED SPIN CONDITIONS FOR TETRABROMOBISPHENOL APOLYCARBONATE HOLLOW FIBER MEMBRANES

In order to form a basis from which comparisons can be made, a set ofstandard spinning and processing conditions are developed. The selectionof this standard set of conditions is based on data from early attemptsto prepare fibers and corresponds to fiber that is easily formed andgives consistent results from one spin run to another. A series offourteen experiments using the standard set of conditions are performedto prepare fibers and the fibers are tested for oxygen and nitrogenpermeation. These permeation and separation factors for these fourteenexperiments are averaged to give a performance standard to measure allother experiments against.

a composition of 52 weight percent tetrabromobisphenol A polycarbonate,32.5 weight percent N-Methyl pyrrolidone (solvent), and 15.5 weightpercent of triethylene glycol (non-solvent), (solvent to non-solventratio of 2.1) is fed into the melt pot of a melt pot extruder. Methylenechloride in an amount equal to about 30 weight percent of the totalcomposition is added to the vessel. The mixture is heated to 95° C. andheld until the mixture is a homogeneous solution. Most of the methylenechloride flashes during this heating step. A nitrogen purge is passedinto the melt pot at 500 cc per minute and nitrogen containingvolatilized methylene chloride is withdrawn from a port in the melt pot.From the melt pot the composition is passed to a transfer line andpumped to the spinnerette at a flow rate of 15 g/min. The transfer lineand spinnerette face are held at a temperature of 75° C. The compositionis extruded into a hollow fiber shape through an annulus of 254 microns(0.01 inch) with an outside diameter of 1727 microns (0.068 inch) with acore gas pin feeding a core gas of nitrogen down the bore at a rate of8.8 standard cubic centimeters a minute. The line speed is 100 ft perminute. The fiber is extruded into an air quench zone of a length of 1foot at ambient temperature. The fiber is passed into a quench bath ofwater at 4° C. with a residence time of 1.7 seconds. The fiber is takenup and thereafter placed into a bath of water at 90° C. for ten minutes.The fibers are hung vertically and dried by passing air over the fibersat a flow of 100 ft/min over the fibers for about two hours. The fibersprepared have a size of 140 ×94 microns (OD ×ID). The membranes preparedin Examples 1 and 14 are examined by photomicrographs and such membraneshave a porous outer surface, a porous inner surface, and have a regionwhich separates oxygen from nitrogen as demonstrated by the separationfactors stated.

PERMEABILITY TESTING PROCEDURE

After the fiber is dried the fibers are tested for permeationproperties. The test device is a pressure vessel with four ports, twotubesheet ports, one feed port through which the compressed gas entersthe vessel, and an exit or purge port through which the compressed gascan be purged from the vessel. Two hundred ten (210) fibers are passedinto one of the tubesheet ports and out the other allowing for a 31.5 cmlength of the fibers to be contained within the test device. Epoxytubesheets are formed in the two tubesheet ports to give a leak-tightbond between the fiber and the two ports. Test units are thenpressurized with nitrogen at 50 psig at allowing compressed nitrogen toenter the test device through the feed port while leaving the exit portclosed. The exit port is then opened for two minutes to purge the vesselof air and then closed with pure nitrogen left in the vessel. With theexit port closed and the feed port opened, the gas contained within thetest device, by means of a pressure driving force, permeates through thewalls of the hollow fibers and passes through the lumen of the fibersand out through the tubesheet ports where the flow rate is measuredeither by means of bubble or mass flow meters. There is negligible backpressure on the gas exiting the tubesheet. After testing with nitrogenthe feed gas is changed to oxygen and the vessel is purged for about twominutes to give pure oxygen at 50 psig in the test device. The amount ofoxygen permeating through the fiber walls is measured by combining theoutputs from the two tubesheet ports. From these flow measurements, thegas permeation rates and separation factor can be calculated by use ofthe following equations. ##EQU1## The units are scc/cm² cmHg sec.

Measured flow = standard cubic centimeters / minute.

Surface area of fibers = 3.14 × OD (outside diameter, cm) × length × thenumber of fibers.

Pressure (cm Hg) = psi × 76 /14.7.

The results are compiled in Table 1.

Separation factor is defined as the Oxygen flux divided by the Nitrogenflux.

                  TABLE 1                                                         ______________________________________                                                                Oxygen/Nitrogen                                                               Separation                                            Example      Oxygen Flux.sup.2                                                                        Factor                                                ______________________________________                                         1           4.8 ± .5                                                                              6.8 ± .1                                            2           7.4 ± .4                                                                              6.4 ± .1                                            3           6.2 ± .1                                                                              6.4 ± .3                                            4           7.6 ± .3                                                                              6.7 ± .1                                            5           7.0 ± .1                                                                              5.9 ± .1                                            6           5.8 ± .2                                                                              6.8 ± .2                                            7           9.0 ± .2                                                                              6.8 ± .2                                            8           8.3 ± .1                                                                              6.7 ± .1                                            9           7.2 ± .1                                                                              6.0 ± .2                                           .sup. 10.sup.1                                                                             4.4 ± .4                                                                              6.0 ± .2                                           11           6.5 ± .3                                                                              6.0 ± .5                                           12           6.1 ± .1                                                                              6.2 ± .1                                           13           7.4 ± .1                                                                              6.5 ± .1                                           14           8.1 ± .1                                                                              6.6 ± .2                                           AVERAGE       7.0 ± 1.1                                                                            6.4 ± .3                                           ______________________________________                                         .sup.1 Not included in average, suspect blend composition                     .sup.2 Units (1 × 10.sup.-6) scc/cm.sup.2 · cmHg .multidot     sec                                                                      

Examples 15-36

Hollow fibers are prepared using the standard conditions describedhereinbefore using several spin compositions. The hollow fibers preparedare tested for oxygen and nitrogen permeability using the proceduredescribed hereinbefore. The various spin compositions and results arecompiled in Table 2. The spin compositions have about 1 to about 6percent residual methylene chloride therein.

Example 35 is performed using some different conditions than the otherexamples. During the blend formation stage the blend is heated to 120°C. The spinnerette temperature is controlled at 70° C. There is a 9 inchair quench zone. The fiber is passed from the quench bath to a leachbath at ambient temperatures. The fiber is exposed to a further bath ofwater for 10 minutes at 80° C. Thereafter the fiber is extracted with amixture of 50/50 mixture of isooctane and isopropanol for one hour. Thefibers are examined by photomicrograph. The membrane has a dense regionof the outer surface and a porous region below the dense region.Therefore the membrane of Example 35 is not an example of the invention.

                  TABLE 2                                                         ______________________________________                                               Polymer                                                                       in Spin  Solvent                 Melt Pot                                     Composi- to Non-          Separa-                                                                              Tempera-                                     tion     solvent   Oxygen tion   ture                                  Example                                                                              wgt %    ratio     Flux   Factor °C.                            ______________________________________                                        15       45     2.1        6 ± .5                                                                            3 ± .4                                                                           85                                    16       45     2.1       2. ± 1                                                                             5 ± .3                                                                           110                                   17       45     2.5       1.7 ± .1                                                                           5 ± .5                                                                           85                                    18       45     2.3        5 ± .9                                                                           2.4    110                                                              2 ± .2                                                                           4.0                                          19       52     2.1       8 ± 1                                                                             6.5 ± .3                                                                          98                                    20       52     2.0       10 ± 1                                                                            6.5 ± .3                                                                          98                                    21       44     1.9       4.2 ± .2                                                                          3.5 ± .2                                                                          80                                    22       44     2.1        4 ± .2                                                                           3.9 ± .3                                                                          80                                    23       44     2.3        2 ± .2                                                                           3.8 ± .1                                                                          80                                    24       50     2.1        7 ±  .1                                                                          5.5 ± .1                                                                          92                                    25       54     2.1       7.3 ± .1                                                                           6 ± .1                                                                           92                                    26       51     1.9       5.4 ± .2                                                                          5.3 ± .2                                                                          95                                    27       51     2.1        7 ± .4                                                                           5.8 ± .2                                                                          95                                    28       53     2.1       3.6 ± .3                                                                          6.3 ± .4                                                                          95                                    29       53     1.9       2.8 ± .4                                                                          5.5 ± .1                                                                          95                                    30       53     1.9       4.8 ± .2                                                                          6.7 ± .1                                                                          95                                    31       52     1.9       9.7 ± .4                                                                          4.3 ± .5                                                                          95                                    32       52     2.0       9.8 ± .1                                                                          6.2 ± .2                                                                          95                                    33       52      2.05     9.1 ± .3                                                                          6.1 ± .1                                                                          95                                    34       52     2.3       *      *      *                                     35       50     NMP only  .021   6.4    105                                   36       52     25        0.2 ± .05                                                                         ***    ***                                   ______________________________________                                         *Fiber did not phase separate                                                 **Composition not spinnable                                                   ***Not measurable, flow rates less than 0.05                             

Examples 37-40

Hollow fibers are prepared from spin compositions containing polymers oftwo different molecular weights than the molecular weights of thepolymer used to set the standard conditions in Examples 1-14. Thepolymer content of the spin composition and the solvent to non-solventratio is described in Table 3. The results are contained in Table 3.

                  TABLE 3                                                         ______________________________________                                        Use of Various Polymers with Different Molecular                              Weights                                                                                                                 Melt                                               %                   Separa-                                                                              Pot                                                Poly-               tion   Temper-                             Example                                                                              M.sub.w.sup.1                                                                         mer     S/NS.sup.2                                                                          Flux  Factor ature                               ______________________________________                                        37     125,000 52      2.1   6.1   2.8    80                                  38     191,000 52      2.10  9.5   4.0    95                                  39     191,000 52      2.13  9.4   6.8    95                                  40     191,000 52      2.16  7.6   6.6    98                                  Standard                                                                             163,000 52      2.1   7.0   6.4    95                                  Condi-                                                                        tions                                                                         ______________________________________                                         .sup.1 The stated values are rounded to the nearest thousand.                 .sup.2 S/NS is the solvent nonsolvent ratio.                             

Examples 41-44

Hollow fibers are prepared using the standard conditions using threedifferent line speeds, and the fibers are tested for oxygen and nitrogenpermeability.

The size of the quench zones are adjusted to keep the residence times ofthe fibers in the baths constant. The conditions are the results arecompiled in Table 4.

                  TABLE 4                                                         ______________________________________                                        FIBER PROPERTIES AS A FUNCTION OF LINE SPEED                                                               Oxygen/                                                                       Nitrogen                                                  Line Speed          Separa-                                                   (Feet/              tion    Fiber Size                               Example  Minute)   Flux      Factor  (Microns)                                ______________________________________                                        41        50.sup.1 5.13 ± .28                                                                           5.44 ± .28                                                                         140 × 94                           42       100.sup.1 7.02 ± .1                                                                            5.85 ± .10                                                                         140 × 94                           43       100.sup.2 5.79 ± .18                                                                           6.80 ± .20                                                                         140 × 94                           44       150.sup.2 5.59 ± .10                                                                           7.21 ± .20                                                                         140 × 94                           ______________________________________                                         .sup.1 Fibers in Examples 41-42 are prepared from the same melt pot run.      .sup.2 Fibers in Examples 43-44 are prepared from the same melt pot run. 

Examples 45-60

Several hollow fibers are prepared using the standard conditions withthe exception that different fiber sizes are prepared. The fibers aretested for oxygen and nitrogen permeabilities. The results are compiledin Table 5.

                  TABLE 5                                                         ______________________________________                                        FIBER PROPERTIES AS A FUNCTION OF FIBER SIZE                                           Fiber Size             Separation                                    Example  (Microns)    O.sub.2 Flux                                                                            Factor                                        ______________________________________                                         45*     204 × 140                                                                            0.5 ± .08                                                                            6.4 ± .4                                   46       140 × 94                                                                             3.4 ± .21                                                                            5.3 ± .5                                   47       158 × 106                                                                            3.8 ± .5                                                                             6.7 ± .1                                   48       140 × 94                                                                             4.8 ± .5                                                                             6.8 ± .1                                   49        112 × 74**                                                                          6.8       6.1                                           50       140 × 94                                                                             8.3 ± .1                                                                             6.7 ± .05                                  51       125 × 85                                                                             7.9 ± .1                                                                             6.5 ± .2                                   52       140 × 94                                                                             4.4 ± .4                                                                             6.0 ± .2                                   53       112 × 74                                                                             7.5 ± .1                                                                             5.6 ± 0.5                                  54       140 × 94                                                                             7.3 ± .3                                                                             5.8 ± .1                                   55       112 × 74                                                                             10.8 ± 5.3 ± .9                                   56       140 × 94                                                                             7.4 ± .1                                                                             6.5 ± .1                                   57        112 × 74**                                                                          11        5.2                                           58       106 × 64                                                                             8.8 ± .3                                                                             5.3 ± .3                                   59       103 × 68                                                                             10.9 ± 1.3                                                                           3.7 ± 1.1                                  60       110 × 74                                                                             10.0 ± .8                                                                            4.0 ± 1.3                                  ______________________________________                                         *The fiber is solvent dried with a 50/50 mix isooctaneisopropyl alcohol       prior to testing                                                              **Result of only one out of four samples                                 

Examples 45-46 are generated from the same melt pot run. Examples 47-49are generated from the same melt pot run. Examples 50-51 are generatedfrom the same melt pot run. Examples 52-53 are generated from the samemelt pot run. Examples 54-55 are generated from the same melt pot run.Examples 56-60 are generated from the same melt pot run.

The fiber size has a significant effect on the permeabilitycharacteristics of the hollow fibers. In general, the smaller the fiberthe higher the intrinsic permeation rate of oxygen, while the separationfactor is fairly insensitive to fiber size.

Examples 61-68

Several hollow fibers are prepared using the standard conditionsdescribed hereinbefore, with the exception that the residence time andthe temperature of the quench bath is altered. The fibers are tested foroxygen and nitrogen permeability. The results are compiled in Table 6.

                  TABLE 6                                                         ______________________________________                                        FIBER PROPERTIES AS A FUNCTION OF QUENCH BATH                                 CONDITIONS                                                                            Residence                    Separa-                                          Time      Temperature        tion                                     Example (Seconds) °C. Flux    Factor                                   ______________________________________                                        61      1.7       5° C.                                                                             8.1 ± .1                                                                           6.0 ± .2                              62      1.7       22° C.                                                                            5.4 ± .2                                                                           6.1 ± .5                              63      1.7       5° C.                                                                             7.0 ± .1                                                                           5.9 ± .1                              64      0.6       5° C.                                                                             7.1 ± .3                                                                           5.4 ± .1                              65      1.7       5° C.                                                                             9.0 ± .2                                                                           6.8 ± .2                              66      0.6       5° C.                                                                             8.4 ± .2                                                                           6.9 ± .2                               67*    1.7       2° C.                                                                             7.8 ± .1                                                                           6.1 ± .1                               68*    1.7       6° C.                                                                             6.1 ± .3                                                                           6.0 ± .1                              ______________________________________                                         *Fiber size held at 140 × 94                                       

Examples 61 and 62, 63 and 64, 65 and 66, and 67and 68, respectively,are from the same melt pot runs.

The residence time in the first liquid quench bath, from 1.7 to 0.6seconds, has little effect on the ultimate performance of the fiber.Temperature has an effect on the fibers gas permeation properties. Asthe temperature is raised from 5° to 22° C., the oxygen permeability islowered. The selectivity of the fiber appears to be unaffected by thistemperature change.

Examples 69-88

Several hollow fibers are prepared using the procedure describedhereinbefore, with the exception that some of the hollow fibers areprocessed through a third bath of water placed between the first andsecond baths. The third bath is held at a temperature of about 20° C.and the residence time is about two minutes. The fibers are tested foroxygen and nitrogen permeability. The results are compiled in Table 7.The total residence time of the fiber in the baths is the same whethertwo or three baths are used.

                  TABLE 7                                                         ______________________________________                                        GAS PROPERTlES OF FIBERS PROCESSED WITH AND                                   WITHOUT A THIRD LIQUID BATH                                                                                   Separa Fiber                                         Percent  Third           tion   Size                                   Example                                                                              Polymer  Bath    Flux    Factor (Microns)                              ______________________________________                                        69     53       Y       4.6 ± 1                                                                            7.0 ± .2                                                                          140 × 94                         70     52       N       4.8 ± 5                                                                            6.8 ± .1                                                                          140 × 94                         71     52       Y       4.0 ± 3                                                                            6.7 ± .1                                                                          158 × 106                        72     52       N       3.8 ± .5                                                                           6.7 ± .1                                                                          158 × 106                        73     52       Y       6.9 ± 4                                                                            5.2 ± .7                                                                          112 × 74                         74     52       N        6.8    6.1    112 × 74                         75     50       Y       6.9 ± .1                                                                           5.5 ± .1                                                                          140 × 94                         76     50       N       7.3 ± .6                                                                           5.1 ± .2                                                                          140 × 94                         77     50       Y       4.6 ± .2                                                                           5.5 ±  .2                                                                         158 × 106                        78     50       N       5.1 ± .5                                                                           5.0 ± .2                                                                          158 × 106                        79     50       Y       12.5 ± .5                                                                          3.4 ± .2                                                                          112 × 74                         80     50       N       14.0    3.5    112 × 74                         81     54       Y       5.1 ± .1                                                                           5.8 ± .2                                                                          140 × 94                         82     54       N       7.3 ± .1                                                                           6.0 ± .1                                                                          140 × 94                         83     54       Y       2.3 ± .1                                                                           5.4 ± .1                                                                          158 × 106                        84     54       N       4.2 ± .4                                                                           5.8 ± .1                                                                          158 × 106                        85     54       Y       20.2 ± 3                                                                           1.4 ± .1                                                                          112 × 74                         86     54       N       23.3 ± 2                                                                           1.4 ± .1                                                                          112 × 74                         87     52       Y       4.8 ± .7                                                                           6.5 ± .1                                                                          140 × 94                         88     52       N       7.4 ± .4                                                                           6.4 ± .1                                                                          140 × 94                         ______________________________________                                    

The presence of a third liquid bath demonstrates its greatest effect inExamples 81-86 where the polymer weight percentage is about 54. This isexhibited primarily in the oxygen permeation rate, with little affect onthe separation factor. Examples 69-74, 75-80, 81-86, and 87-88,respectively, are prepared from the same melt pot run.

Example 89

A hollow fiber is prepared using the standard procedure with theaddition of the third liquid bath, and the fibers are analyzed forresidual solvent, and non-solvent after each bath. The temperature andresidence in the third bath is about the same as the third bath inExamples 69 to 88. The total residence time of the fiber is the threebaths is the same as where two baths are used. The results are compiledin Table 8.

                  TABLE 8                                                         ______________________________________                                        PLASTICIZER CONTENT OF FIBER AT VARIOUS                                       STAGES OF PROCESSING                                                                                           PERCENT                                               PERCENT     PERCENT     METHYLENE                                    STAGE    NMP         TEG         CHLORIDE                                     ______________________________________                                        Pre-     32.5        15.6        3.0                                          extrusion                                                                     After first                                                                            15.7        7.5         0                                            bath                                                                          After, second                                                                           5.0        0           0                                            bath                                                                          After third                                                                             0.7        0           0                                            bath                                                                          ______________________________________                                         Note:                                                                         Half of the solvent and nonsolvent are removed in the air quench zone and     first liquid quench (the bath temperature is 4.5° C., and the          residence time is 1.7 seconds). The fiber size is 140 × 94 microns.

Examples 90-105

Several hollow fibers are prepared using the standard conditionsdescribed hereinbefore, with the exceptions that the residence time andtemperature of the second liquid bath are altered.

                  TABLE 9                                                         ______________________________________                                        FIBER PROPERTIES AS A FUNCTION OF SECOND                                      LIQUID BATH CONDITIONS                                                                                  Percent                                                                       Residual                                                    Temper-           Solvent       Separa-                                       ature    Time     and Non-      tion                                  Example °C.                                                                             (Minutes)                                                                              solvent                                                                              Flux   Factor                                ______________________________________                                         90     90       10       <1     3.1 ± .1                                                                          6.6 ± .2                            91     90        1/2     0      4.9 ± .4                                                                          6.4 ± .1                            92     90        1       0      5.0 ± .3                                                                          6.4 ± .3                            93     90        5       0      7.0 ± .2                                                                          6.5 ± .1                            94     90       10       0      7.4 ± .4                                                                          6.4 ± .1                            95     90       10       0      5.8 ± .2                                                                          6.8 ± .2                            96     70        1       0      3.9 ± .4                                                                          7.0 ± .1                            97     70        5       0      4.4 ± .1                                                                          7.3 ± .2                            98     70       10       0      5.0 ± .2                                                                          6.8 ± .1                            99     90       10       0      9.0 ± .2                                                                          6.8 ± .2                           100     90       10       *3     6.3 ± .2                                                                          7.0 ± .3                           101     70       10       0      7.1 ± .1                                                                          7.0 ± .2                           102     70       10       *3     4.2 ± .2                                                                          7.3 ± .2                           103     90       10       0      7.3 ± .3                                                                          5.8 ± .1                           104     90       10       *1     6.5 ± .3                                                                          5.9 ± .5                           105     90       10       *2     5.7 ± .2                                                                          6.3 ± .1                           ______________________________________                                         *Liquid bath has the described solvent percentages added thereto.        

Examples 91-94, 95-98, 99-102, and 103-105, respectively, are preparedfrom the same melt pot run.

The gas permeability is affected by the conditions of the second liquidbath. Higher temperature and residence time result in higher gaspermeability. Higher solvent content in the bath result in lowering thegas permeability significantly while the separation factor risesslightly.

Examples 106-111

Several hollow fiber membranes are prepared wherein the time periodbetween the first bath and the second bath is varied. The fibers aretested for oxygen and nitrogen permeability. The results are compiled inTable 10. The membrane of Example 106 is examined by photomicrograph andthe membrane exhibits a porous outer and a porous inner surface.

                  TABLE 10                                                        ______________________________________                                        FIBER PROPERTIES RESULTING FROM DELAY                                         BETWEEN THE TWO BATHS                                                                Time                                                                          Between                       Fiber                                           Baths                         Size                                     Example                                                                              (Minutes)  O.sub.2 Flux                                                                           Selectivity                                                                             (Microns)                                ______________________________________                                        106     0         8.1 ± .1                                                                            6.0 ± .2                                                                             140 × 94                           107    30         7.6 ± .3                                                                             6.7 ± .05                                                                           140 × 94                           108     0         7.5 ± .1                                                                            5.6 ± .1                                                                             125 × 85                           109    15         7.0 ± .1                                                                            6.1 ± .1                                                                             125 × 85                           110    45         8.0 ± .3                                                                            6.7 ± .2                                                                             125 × 85                           111    60         7.5 ± .1                                                                            6.0 ± .3                                                                             125 × 85                           ______________________________________                                    

Examples 106-107 and 108-111, respectively, are from the same melt potrun.

Examples 106-111 demonstrate the fibers experience no deleteriouseffects due to longer times between the baths. In fact, the use of sucha delay may be beneficial.

Examples 112-114

Two spin runs are performed wherein the fibers are dried right afterremoval from the second liquid bath. A third spin run is performedwherein the fibers are stored in water for 20 hours between the secondbath and the drying step. The fibers are tested for oxygen and nitrogenpermeability. The results are compiled in Table 11.

                  TABLE 11                                                        ______________________________________                                        EFFECT OF DELAY IN DRYING AFTER SECOND BATH                                           Water    Percent                                                              Storage  Solvent            Separa-                                           Time     (In Second         tion                                      Example (Hours)  Bath)       Flux   Factor                                    ______________________________________                                        112     0        0           5.1 ± .2                                                                          6.3 ± .3                               113     0        2           1.3 ± .1                                                                          6.1 ± .1                               114     20*      0           7.3 ± .3                                                                          5.8 ± .1                               ______________________________________                                    

Examples 115-124

Several hollow fibers are prepared with differing amounts of residualN-methylpyrrolidone (NMP) in the final fibers. The fibers are tested foroxygen and nitrogen permeability. The membrane of Example 115 isexamined by photomicrograph and the membrane exhibits a porous outer anda porous inner surface. The results are compiled in Table 12.

                  TABLE 12                                                        ______________________________________                                        EFFECT OF RESIDUAL SOLVENT IN THE                                             FIBERS ON PERMEATION                                                                                              Fiber                                                                 Selectiv-                                                                             Size                                      Example                                                                              Percent NMP                                                                              O.sub.2 Flux                                                                            ity     (Microns)                                 ______________________________________                                        115    0.76       7.0 ± .3                                                                             5.8 ± .4                                                                           140 × 94                            116    3.70       1.0 ± .4                                                                             6.3 ± .4                                                                            204 × 147                          117    1.06       3.9 ± .4                                                                             6.7 ± .1                                                                            158 × 106                          118    0.41       4.7 ± .4                                                                             6.9 ± .1                                                                           140 × 94                            119    0.35       6.9 ± .4                                                                             5.6 ± .4                                                                           112 × 74                            120    1.80       4.9 ± .4                                                                             6.4 ± .4                                                                           140 × 94                            121    1.54       5.0 ± .3                                                                             6.4 ± .3                                                                           140 × 94                            122    1.20       7.0 ± .2                                                                             6/5 ± .1                                                                           140 × 94                            123    1.03       7.4 ± .4                                                                             6.4 ± .1                                                                           140 × 94                             124*  1.14       4.8 ± .7                                                                             6.5 ± .1                                                                           140 × 94                            ______________________________________                                         *A third liquid bath at 20° C. is used with a residence time of tw     minutes.                                                                 

The solvent content of the fiber prepared by the process correlates wellwith the intrinsic gas permeation rates of the fiber. As the residualsolvent in the fiber increases, the permeation rate decreases.

Examples 125

Tetrabromobisphenol A polycarbonate is tested for solubility in severalsolvents and non-solvents. Weighed amounts of polymer and liquid areplaced in 4 dram-capacity glass vials with polyethylene-lined caps.About 2.5 grams of liquid is usually used. Initial polymer concentrationis about 5 weight percent. The vials are placed on mechanical rollersfor at least 24 hours or until complete solution is affected. Additionalpolymer, if indicated, is added to prepare concentrations of about 10,25, and 50 weight percent. Insoluble mixtures with liquid boiling pointsin excess of about 100° C. are placed in a 100° C. forced-air oven forat least 24 hours' observation or until solution is completed. Thepolymer is arbitrarily designated as being "insoluble" in the liquid if5 weight percent or less dissolved; "moderately" soluble if 5-25 percentdissolved; and "soluble" if more than 25 percent dissolved. The resultsare compiled in Table 13.

                  TABLE 13                                                        ______________________________________                                        SOLUBILITY OF TETRABROMOBISPHENOL A IN                                        VARIOUS SOLVENTS                                                                                Relative                                                    Compound          Solubility*                                                 ______________________________________                                        poly(dimethyl-    I<0.8% b f                                                  siloxane) 50 cs.                                                              perfluoro(methyl- I<1.4% b                                                    cyclohexane)                                                                  hexane            I<1.6% b                                                    triethylamine     I<4.7% b                                                    butyl stearate    I<4.9% b f                                                  methylcyclohexane I<4.6% b f                                                  dioctyl phthalate I<4.7% b f                                                  dodecane          I<4.7% b f                                                  isopropylcyclo-   I<4.95% b f                                                 hexane                                                                        t-butylcyclohexane                                                                              I<4.9% b f                                                  hexadecane        I<4.8% b f                                                  diisopropyl ketone                                                                              I<4.9% b f                                                  cyclohexane       I<4.8% b                                                    bis(2-methoxyethyl                                                                              S>50.3% b                                                   ether)                                                                        ethyl benzoate    S>25.1<50.1% bcg@f                                          diethylene glycol I<4.9% b f                                                  dibutyl ether                                                                 triethyl          I<4.5% b f                                                  orthoformate                                                                  methyl isobutyl   I<4.7% b f c                                                ketone                                                                        tricresyl phosphate                                                                             I<5.0% b>5.0% f                                             methyl myristate  I<4.9% b f                                                  triethylene glycol                                                                              S>50.4% b                                                   dimethylether                                                                 n-octyl acetate   S>50.1% b                                                   dicyclohexyl      I<4.8% b f                                                  methyl laurate    I<4.7% b f                                                  tetraethylene     S>50.3% b                                                   glycol                                                                        dimethylether                                                                 carbon            I<4.7% b                                                    tetrachloride                                                                 n-propylbenzene   I<4.9% b f c                                                methyl stearate   I<4.7% e f                                                  piperidine        S>26.3% b f d                                               xylene            I<5.5% bc<5.5% f                                            decahydronaphtha- I<4.4% b f                                                  lene (cis & trans)                                                            ethylbenzene      I<4.9% b f c                                                diethyl ketone    S>50.2% b                                                   toluene           I<4.5% b f c                                                N--ethylmorpholine                                                                              S>50.1% b                                                   cyclohexyl acetate                                                                              S>50.5% b                                                   butyraldehyde     I<4.8% b                                                    tetrahydrofuran   S>51.4% b                                                   ethyl acetate     I<4.7% b c                                                  isophorone        S>25.3<50.1b>50.1f                                          cyclohexylbenzene I<4.8% b f                                                  trichloroethylene S>50.2% b c                                                 diacetone alcohol I<4.9% b f                                                  1,2,4-trichloro-  S>25.4<50.1bc?>50f                                          benzene                                                                       perchloroethylene I<4.9% b f                                                  chloroform        S>50.8% b c                                                 methyl ethyl      S>50.1% b c?                                                ketone                                                                        styrene           I<4.7% b c                                                  ethyl formate     I<5.0% b c                                                  benzaldehyde      S>50.1% b f                                                 tetrahydro-       I<4 8% b f                                                  naphthalene                                                                   chlorobenzene     S>50.4% b c g@f                                             methyl acetate    I<4.8% b c                                                  methylene chloride                                                                              S>51.1% b c                                                 acetone           I<4.6% b c                                                  cyclohexanone     S>50.3% b                                                   1-cyclohexyl-2-   I<4.7% b>4.7% f                                             pyrrolidinone                                                                 o-dichlorobenzene S>50.1% b c g@f                                             epsilon-          S>25.3<50.1b>50.1f                                          caprolactone                                                                  phenyl ether      S>50.1% e f c?@b                                            methyl formate    I<5.0% b                                                    methyl iodide     S>50.2 b                                                    cyclopentanone    S>50.3 b                                                    hexamethyl-       I<4.9% b>4.9% f                                             phosphoramide                                                                 methyl benzoate   S>50.5% b f c?@b                                            styrene oxide     S>50.5% b f c?@b&f                                          1-ethyl-2-        S>50.1% b                                                   pyrrolidinone                                                                 acetophenone      S>50.1% b                                                   methyl salicylate S>25.6%<50.1b>50.1f                                         1,1,3,3-          S>50.3 b c g@f                                              tetramethylurea                                                               1-bromonaphthalene                                                                              S>25.3<50.0% bfc                                            1-hexanol         I<4.7% b f                                                  dimethyl phthalate                                                                              I<4.9% b>4.9% f                                             pyridine          S>50.1% b                                                   N,N--dimethyl-    S>50.2% b                                                   acetamide                                                                     propionitrile     I<4.9% b c                                                  triethyl phosphate                                                                              I<4.8% bc?d?4.8% f                                          dimethyl          I<4.8% b f                                                  malonate                                                                      polyethylene      I<2.2% b f                                                  glycol E400                                                                   1-acetyl-         S>50.1% b                                                   2-furaldehyde     S>50.1% b                                                   N--methyl-        S>50.2% b                                                   pyrrolidinone                                                                 1-benzyl-2-       S>25.9<50.1b>50.1f                                          pyrrolidone                                                                   2-propanol        I<2.9% b                                                    1-formyl-         S>50.1% b                                                   piperidine                                                                    diiodomethane     S>25.2% b f                                                 acetonitrile      I<4.9% b                                                    dimethyl-         M=<14.1% b f c                                              sulfoxide                                                                     N,N--dimethyl-    S>55.0% b                                                   formamide                                                                     gamma-            S>50.2% b                                                   butyrolactone                                                                 ethanol           I<3.9% b                                                    nitromethane      1<5.0% b f                                                  N--formyl-        S>25.6<50.2b>50.2f                                          morpholine                                                                    sulfolane         I<4.6% e>4.6% f                                             methanol          I<1.5% b                                                    N--methyl-        I<4.6% e f                                                  acetamide                                                                     2-pyrrolidinone   S>25.8<50.1b>50.1f                                          diethyl ether     I<4.6% b                                                    ethylene glycol   I<5.3 b c                                                   diethyl ether                                                                 ethylene glycol   S>51.0% b                                                   dimethyl ether                                                                ethylene          I<5.0% e f                                                  carbonate                                                                     malonitrile       I<4.9% e f                                                  N--methyl         I<5.0% b f                                                  formamide                                                                     ______________________________________                                         *I = Insoluble: <=5%; M = Moderately Soluble: 5-25%; S = soluble: >25; b      at room temperature; c = insoluble fraction and/or solvated polymer and/o     solventinduced order; d = reacts with polymer; e = at 50° C.; f =      at 100° C.; g = clear.                                            

The behavior of about 27 compounds are marked by a "c". Such behaviorincludes (a) partial dissolution followed by opacification and whiteningof the clear swollen polymer accompanied by cessation of furtherdissolution; this behavior is frequently accompanied by a hazy or cloudysupernatant liquid; (b) dissolution to give a clear solution followed byprecipitation at the same temperature of white and opaque solid, mushygel-like formation, or, at the extreme, a solidification to a "candlewax-like" solid; and (c) dissolution at elevated temperature followed byprecipitation of solid, "gelation", and/or a hazy-cloudy formation inthe supernatant liquid upon cooling. Seven particularly severe cases ofthis behavior are noted. Methylene chloride solutions containing about51% polymer become hard candle wax-like solids after about 17 days'standing at room temperature. DMSO solutions contain about 14 percentpolymer are readily formed at room temperature; they change to a whiteopaque slush after about 36 hours. Redissolution does not occur atelevated temperature. Chloroform solutions containing about 51% polymerare clear at room temperature but changed into candle wax-like solidsafter about 14 days. Chlorobenzene solutions containing about 50%polymer become clear stiff gels after about 11 days at room temperature.The gels become clear at 100° C. but become cloudy when cooled.Tetramethylurea containing about 50% polymer is clear and soluble atroom temperature but becomes a rigid gel after about 8 days. The gelbecomes clear at 100° C.; the clear solution becomes cloudy when cooledto room temperature. A clear solution of about 50% polymer in ethylbenzoate becomes a rigid, opaque gel after 11 days at room temperature.n-Propylbenzene dissolved less than 4.9% polymer at room temperature;solubility is almost complete at 100° C. The warm solution becomes acandle-like solid when cooled to room temperature.

Table 13 indicates several solvents and non-solvents which are goodcandidates for solvent non-solvent pairs useful for spinningpolycarbonate membranes by the process disclosed herein.

Examples 126-129

Fibers are prepared using the same conditions as described in Example 1,with the exception that a polyethylene glycol with a molecular weight of400 is used as the non-solvent, and the solvent non-solvent ratio isadjusted. Fibers are produced with a porous inner surface and a porousouter surface. After the fibers are prepared, the fibers are immersed ina solution of 25 percent by volume of methanol in water for two hours.The fibers are tested as described in Example 1 both before and afterthe immersion in the methanol and water solution, and the results arecompiled in Table 14.

                  TABLE 14                                                        ______________________________________                                        Use of Polyethylene Glycol as a Non-solvent                                                                   Oxygen Separa-                                       Solvent                  Flux   tion                                          Non-              Separa-                                                                              After  Factor                                        solvent  Oxygen   tion   Immer- Immer-                                 Example                                                                              Ration   Flux     Factor sion   sion                                   ______________________________________                                        126    2.10     4.0      7.2    7.5    7.3                                    127    1.95     5.5      7.1    10.0   7.4                                    128    1.80     5.9      7.5                                                  129    1.60     7.1      2.4                                                  ______________________________________                                    

Examples 130-145

Fibers are prepared using the same conditions as described in Example 1,with the exception that different nonsolvents are used, and the solventnon-solvent ratio is varied. Fibers are produced with a porous innersurface and a porous outer surface. After the fibers are prepared, thefibers prepared in Examples 135, 141, 143, and 145 are immersed in asolution of 25 percent by volume of methanol in water for two hours. Thefibers are tested as described in Example 1 and the results are compiledin Table 15. Those examples where the fibers are immersed in a methanolsolution are tested after the immersion in the methanol solution.Membranes of Examples 133 to 135 are examined by photomicrograph and themembranes exhibit porous outer surfaces and porous inner surfaces.

    ______________________________________                                        Use of Varied Nonsolvents                                                                      Solvent                                                      Ex-              Non-            Speara-                                      am-              solvent  Oxygen tion   Fiber                                 ple  Non-solvent Ratio    Flux   Factor Size                                  ______________________________________                                        130  polyethylene                                                                              1.8      3.31 ±                                                                            3.42 ±                                                                            140 × 90                             glycol 1450          .15    .32                                          131  polyethylene                                                                              1.8      2.45   5.45   162 × 104                            glycol 1450                                                              132  ethylene glycol                                                                           3.1      9.3    1.04   140 × 90                        133  ethylene glycol                                                                           3.5      .35 ±                                                                             7.8 ±                                                                             140 × 90                                                  .08    1.6                                          134  ethylene glycol                                                                           3.5      .10 ±                                                                             5.1 ±                                                                             204 × 140                                                 .01    0.1                                          135  ethylene glycol                                                                           3.5      1.5 ±                                                                             6.7 ±                                                                             140 × 90                                                  .1     0.1                                          136  ethylene glycol                                                                           4.5      0.04   5.8    140 × 90                        137  ethylene    1.5      0.074  >7     140 × 90                             carbonate                                                                138  ethylene    1.5      0.104  8.3    161 × 104                            carbonate                                                                139  ethylene    1.5      0.10   6.7    125 × 80                             carbonate                                                                140  ethylene    1.1      0.08   >6     140 × 90                             carbonate                                                                141  ethylene    1.1      0.16   8.7    140 × 90                             carbonate                                                                142  ethylene    1.1      0.05   >6     161 × 104                            carbonate                                                                143  ethylene    1.1      0.17   >6     161 × 104                            carbonate                                                                144  ethylene    1.1      0.68   6.15   110 × 74                             carbonate                                                                145  ethylene    1.1      20 ±                                                                              1.0    110 × 74                             carbonate            10                                                  ______________________________________                                         Polyethylene glycol 1450 is a polyethylene glycol with a molecular weight     of about 1450.                                                           

Examples 146-148

Three membranes are prepared using the conditions described in Examples1-14, and the resultant membranes are examined by photomicrograph. Allof the membranes exhibit a porous outer surface and a porous innersurface. The membranes are tested for permeability properties, theresults are compiled in Table 15.

                  TABLE 15                                                        ______________________________________                                                      Oxygen   Separation                                             Example       Flux     Factor                                                 ______________________________________                                        146           5.5 ± .2                                                                            6.6 ± .2                                            147           8.0 ± .1                                                                            6.0 ± .2                                            148           9.0 ± .3                                                                            6.8 ± .2                                            ______________________________________                                    

All of the membranes prepared examined by photomicrograph demonstrate aporous outer surface and a porous inner surface, except the one examplenoted. It is believed that all of the other membranes prepared have asimilar structure. Examination of the fibers by the naked eye duringfiber spinning indicates that all of the fibers not examined byphotomicrograph look like those examined by photomicrograph.

What is claimed is:
 1. A semi-permeable membrane which comprises apolymeric matrix with two porous surfaces and a region which functionsto separate one or more gases from one or more other gases; wherein thetwo porous surfaces and the region which functions to separate one ormore gases from one or more other gases are comprised of the samepolymeric material.
 2. The membrane of claim 1 wherein the polymericmatrix comprises a polyimide, polycarbonate, polyester,polyestercarbonate, polysulphone, polyethersulphone, polyamide,polyphenylene oxide, or polyolefin.
 3. The membrane of claim 2 whereinthe polymeric matrix comprises a polyester, a polycarbonate, or apolyester carbonate.
 4. The membrane of claim 3 wherein the polymericmatrix comprises a polycarbonate.
 5. The membrane of claim 4 wherein thepolymeric matrix comprises a polycarbonate derived from a bisphenolwherein at least 25 percent of the bisphenol moieties in the backbone ofthe polymer are tetrahalogenated wherein the halogen is chlorine orbromine.
 6. The membrane of claim 5 which separates oxygen fromnitrogen.
 7. A semi-permeable membrane which comprises a polymericmatrix with two porous surfaces and a region which functions to separateoxygen from nitrogen, wherein the polymeric matrix comprises apolycarbonate derived from a bisphenol wherein at least 25 percent ofthe bisphenol moieties in the backbone of the polymer areterahalogenated wherein the halogen is chlorine or bromine, wherein theseparation factor for oxygen and nitrogen is 6.1 or greater.
 8. Themembrane of claim 7 which is in hollow fiber form.
 9. The membrane ofclaim 8 which exhibits a flux of 3.0 ×10⁻⁶ scc/cm² cmHg sec. or greater.10. A semi-permeable membrane which comprises a a polymeric matrix inhollow fiber form wherein the inner surface and the outer surface of thehollow fiber are porous and the hollow fiber membrane is capable ofseparating one or more gases from one or more other gases.
 11. Themembrane of claim 10 wherein the polymeric matrix comprises a polyimide,polycarbonate, polyester, polyestercarbonate, polysulphone,polyethersulphone, polyamide, polyphenylene oxide, or polyolefin. 12.The membrane of claim 11 wherein the polymeric matrix comprises apolyester, a polycarbonate, or a polyester carbonate.
 13. The membraneof claim 12 wherein the polymeric matrix comprises a polycarbonate. 14.The membrane of claim 13 wherein the polymeric matrix comprises apolycarbonate derived from a bisphenol wherein at least 25 percent ofthe bisphenol moieties in the backbone of the polymer aretetrahalogenated wherein the halogen is chlorine or bromine.
 15. Themembrane of claim 14 which separates oxygen from nitrogen.
 16. Asemi-permeable membrane which comprises a polymeric matrix in hollowfiber form wherein the inner surface and the outer surface of the hollowfiber are porous and the hollow fiber membrane is capable of separatingoxygen from nitrogen with a separation factor for oxygen and nitrogen is6.1 or greater, wherein the polymeric matrix comprises a polycarbonatederived from a bisphenol wherein at least 25 percent of the bisphenolmoieties in the backbone of the polymer are tetrahalogenated wherein thehalogen is chlorine or bromine.
 17. The membrane of claim 16 whichexhibits a flux of 3.0 ×10⁻⁶ scc/cm² cmHg sec. or greater.
 18. A processfor preparing a semi-permeable membrane with two porous surfaces andwherein the membrane is capable of separating one or more gases from oneor more other gases which comprises:(A) forming a mixture whichcomprises a polymer which is capable of being formed into a membrane, asolvent for the polymer, and a non-solvent for the polymer, wherein saidmixture has sufficient viscosity at extrusion temperatures to retain thedesired membrane shape; (B) heating the mixture to a temperature atwhich the mixture forms a homogeneous fluid and is extrudable; (C)extruding the mixture in the desired membrane shape wherein theextrusion is performed under conditions such that the mixture is closeto the phase boundary of the phase diagram for the mixture; and (D)passing the formed membrane through one or more quench zones wherein themixture phase separates, and the major portion of the solvent andnon-solvent are removed from the formed membrane, where at least one ofthe quench zones comprises a liquid which has low solubility in thepolymer;wherein the membrane formed has two porous surfaces with adiscriminating region capable of separating oxygen from nitrogen.
 19. Aprocess for preparing semi-permeable membrane comprisingtetrahalogenated bisphenol polycarbonate with two porous surfaces andwhich is capable of separating one or more gases from one or more othergases which comprises:(A) forming a mixture comprising(i) a bisphenolpolycarbonate wherein at least 25 percent by weight of the bisphenolmoieties are tetrahalogenated wherein the halogen is chlorine orbromine; (ii) a solvent for the polycarbonate which comprises a glycolether which corresponds to the formula R³ O--(CH₂ CH₂ O)_(r) --R³wherein R³ is methyl or ethyl, and r is an integer of between about 1and 20; a dialkyl ketone wherein the alkyl groups independently aremethyl or ethyl; morpholine substituted on the nitrogen atom with analkyl, formyl or alkanoyl moiety; pyrrolidinone or N--C₆₋₄ alkyl,N--C₅₋₆ cycloalkyl, or N--C₆₋₁₀ aryl or alkaryl substitutedpyrrolidinone; C₁₋₄ alkoxycarbonyl, formyl, nitro, or halo substitutedbenzene; tetrahydrofuran; dimethyl formamide; cyclohexanone;N,N-dimethyl acetamide; acetophenone; methylene chloride; sulfolane;cyclohexyl acetate; 1,1,3,3-tetramethylurea; isophorone; caprolactone;1-formylpiperidine; methyl salicylate; hexamethylphosphoramide; phenylether; or bromonaphthalene; and, (iii) a non-solvent for thepolycarbonate which comprises a glycol or glycol ether which correspondsto the formula R⁴ O--(CH₂ CH₂ O)_(q) --R⁴ wherein R⁴ is independently ineach occurrence hydrogen or C₁₋₄ alkyl, and q is an integer or about 1to about 250; an ester corresponding to the formula R⁵ COOR⁶ wherein R⁵is hydrogen or C₁₋₁₉ alkyl, and R⁶ is C₁₋₁₀ alkyl; a C₁₋₁₀ alkanol;cyclohexane, unsubstituted or substituted with an alkyl, cycloalkyl, orperfluoroalkyl moiety; a C₅₋₂₀ alkane; a dialkyl ketone wherein at leastone of the alkyl moieties is C₃ or greater; an amide corresponding tothe formula R⁷ CONHR⁸ wherein R⁷ is hydrogen or C₁₋₁₀ alkyl and R⁸ isC₁₋₁₀ alkyl; an acetyl or C₁₋₁₀ alkyl nitrile; acetone; a C₁₋₁₀ alkylaldehyde; a trialkyl amine; nitromethane; trialkyl orthoformate;diacetone alcohol; dimethyl malonate, decahydronaphthalene;tetrahydronaphthalene; malononitrile; dicyclohexyl; ethylene carbonate;sulfolane; alkyl or cycloalkyl substituted benzene; or water; (B)heating the mixture to a temperature at which the mixture is ahomogeneous fluid and extrudable; (C) extruding the heated mixture intoa shape suitable for membrane use wherein the extrusion is performedunder conditions such that the mixture is close to the phase boundary ofthe phase diagram for the mixture; (D) passing the formed membranethrough one or more quench zones wherein the mixture phase separates,and the major portion of the solvent and non-solvent are removed fromthe formed membrane, where at least one of the quench zones comprises aliquid which has low solubility in the polymer; andwherein the membraneformed has two porous surfaces and is capable of separating oxygen fromnitrogen.
 20. The process of claim 19 wherein the mixture is passed fromthe extruder into an air quench zone, under conditions such that aportion of the solvent is removed from the mixture, and from the airquench zone the mixture is passed into one or more liquid quench zonesunder conditions such that the fiber substantially completes phaseseparation and the solvent and non-solvent are substantially removedfrom the mixture in said liquid quench zones.
 21. The process of claim20 wherein the mixture is passed through two liquid quench zones whereinthe mixture is passed through the first liquid zone under conditionssuch that the mixture undergoes significant phase separation in saidbath, and the fiber is passed through the second liquid bath underconditions such that the solvent and non-solvent are substantiallyremoved from the mixture and phase separation is substantiallycompleted.
 22. The process of claim 21 wherein the liquid quench zonescomprise lower alcohols, water, fluorocarbons, lower aliphatichydrocarbons, or mixtures thereof.
 23. The process of claim 22 whereinthe liquid quench zones comprise water.
 24. The process of claim 23wherein 100 weight percent of the bisphenol moieties present aretetrahalosubstituted with chloro or bromo groups.
 25. The process ofclaim 24 wherein at least 50 weight percent of the bisphenol moieties inthe discriminating layer are tetrabromosubstituted.
 26. The process ofclaim 25 wherein the bisphenol is tetrabromobisphenol A.
 27. The processof claim 26 wherein the solvent is N-methyl pyrrolidone, ethylene glycoldimethyl ether, tetrahydrofuran, diethylene glycol dimethyl ether,acetophenone, methylene chloride, or cyclohexanone; and the non-solventis water, diisopropyl ketone, tetraethylene glycol dimethyl ether,diethylene glycol dibutyl ether, hexadecane, diethylene glycol,triethylene glycol, polyethylene glycol with a molecular weight of up toabout 1450, 2-ethoxyethanol, carbon tetrachloride, or dodecane.
 28. Theprocess of claim 27 wherein the solvent non-solvent pair is N-methylpyrrolidone and triethylene glycol, N-methyl pyrrolidone andpolyethylene glycol with a molecular weight of up to about 1450,ethylene glycol dimethyl ether and water, tetrahydrofuran and water,ethylene glycol dimethyl ether and diisopropyl ketone, tetrahydrofuranand diisopropyl ketone, diethylene glyco dimethyl ether and water,diethylene glycol dimethyl ether and tetraethylene glycol dimethylether, acetophenone and diethylene glycol dibutyl ether, methylenechloride and carbon tetrachloride, or acetophenone and hexadecane. 29.The process of claim 28 wherein the solvent non-solvent pair is N-methylpyrrolidone and triethylene glycol, or N-methyl pyrrolidone andpolyethylene glycol with a molecular weight of up to about
 400. 30. Theprocess of claim 22 wherein the membrane shape is a hollow tube, asheet, or a hollow fiber.